In most industrial fields it is desirable to find high performance insulation systems for keeping the fluids conveyed in pipework at constant temperature, so as to make it possible to transfer fluids between equipments that are considerable distances apart, e.g. of several hundreds of meters or even of several kilometers. Such distances are frequent in industries such as oil refineries, liquefied natural gas installations (at −165° C.), and undersea oil fields, extending over several tens of kilometers. Such oil fields are being developed in ever increasing depths of water, where depths can significantly exceed 3000 meters (m).
Numerous systems have been developed to reach a high degree of thermal performance, and specific versions have been developed to operate in satisfactory manner at great depths, i.e. to withstand the pressure at the bottom of the sea. The highest performance technologies that have been developed for this purpose are known as “pipe-in-pipe” or “PiP”, in which an inner pipe conveys the fluid and an outer pipe disposed coaxially about the inner pipe comes into contact with the surroundings, i.e. the sea. The annular space between the two pipes may be filled with an insulating material, or it may be evacuated of all gas.
In very great depths, i.e. 2000 m or 3000 m, or even more, the weight of pipes increases very quickly since the outer pipe must withstand the pressure and as a result it is very thick. Thus, laying such pipe requires very heavy equipment, since in the catenary configuration taken up by the pipe while it is being laid, not only does the length of said catenary increase with increasing depth, but also its unit weight, since the outer pipe must withstand a greater implosion pressure, thereby leading to very high levels of tension in the pipe, at the laying tower installed on the ship, which tensions can reach 500 (metric) tonnes to 700 tonnes, or even 1000 tonnes or more.
In that type of pipe, the annular space, optionally filled with an insulating material, is generally at an absolute pressure that is lower than atmospheric pressure, or is even evacuated of all gas, and to a first approximation it can be considered that the inner pipe must withstand radially the bursting pressure due to the internal fluid, while the outer pipe withstands the implosion pressure created by the hydrostatic pressure at bottom level (ρ.g.h) which is about 1 megapascal (MPa) per 100 m of depth of water, i.e. 30 MPa at a depth of 3000 m. An effect of axial expansion or longitudinal stretching due to pressure, known as the “bottom effect” acts on the circular section of the outer and inner pipes and parallel to the axis of said pipes, and is shared, to a first approximation, by both pipes (because they are interconnected at their ends), prorata their respective sections of material, generally steel.
The internal pressure P within a pipe leads firstly to a “bottom effect” or “piston effect” that is characterized by an axial longitudinal force tending to stretch or longitudinally expand said pipe, and it has the value Fi=P×s, where s is the section of the pipe. Secondly, the internal pressure generates a “radial effect” tending to cause the pipe to burst, with the force acting on a unit length of the wall having the value Fe=π×R×P, where R represents the inside radius of the pipe.
The inner pipe must therefore also be strong and thus present increased thickness when laying at great depths in order to be able to withstand the bottom effect and the bursting radial effect.
Thus, the first problem posed by the present invention is to provide a PiP type insulated pipe of lighter weight so as to present weight per unit length that is considerably reduced, while continuing to present mechanical reinforcement characteristics that ensure that its fatigue behavior is capable of withstanding the stresses generated during laying at great depth.
In installations at great depth, undersea pipes and undersea coaxial pipe assemblies are assembled on land to constitute elements of unit length, of the order of 20 m to 100 m depending on the holding capacity of the laying system. They are then transported in that form out to sea on a laying ship. During laying, the unit lengths of the various coaxial pipe assembly elements are connected to one another on board the ship while they are being laid at sea. It is thus important for the connection operation to be capable of being integrated in the method of assembling and laying the pipe on the sea bottom while slowing the method down as little as possible, in other words the operation must be capable of being performed quickly and easily.
When laying a conventional PiP at great depth, said PiP is subjected to bending, mainly in its bottom portion close to the sea bottom. The bending is at its maximum at the point of contact with the bottom since the radius of curvature decreases from the surface down to the point of contact with the bottom, where it is then at a minimum, after which the PiP rests substantially horizontally on the sea bottom so as to present a radius of curvature that is infinite. The bending generated during laying generates high levels of stress in each of the tubes of the PiP and in the zone interconnecting two successive lengths of PiP.
Junction parts are used in the form of connecting steel forgings that are assembled to the ends of said coaxial pipe assembly elements that are to be assembled together. The junction forging at the downstream end of a first coaxial pipe assembly element that has not yet been assembled is connected to the upstream free end junction forging of a second coaxial pipe assembly element that has already been assembled downstream.
Patents GB-2 161 565 and GB-2 191 842 describes a PiP and its method of assembly, and give two embodiments of junction or connection pieces made of forged steel, the first patent GB-2 161 565 describing a forging made as a single piece, and the second patent GB-2 191 842 describing a forging made up of two elements with the junction between the two elements of two junction forgings being provided by a screw thread, said thread being glued in order to provide sealing.
In both configurations, the forging has two circularly symmetrical branches comprising an outer branch and an inner branch defining between them an annular space, i.e. forming a fork with free cylindrical ends that are assembled to the cylindrical ends respectively of the outer and the inner pipes.
Nevertheless, in both embodiments, shortcomings are observed in the mechanical reliability of the connection of unit lengths of coaxial pipe assembly fitted at their ends with such junction or connection forgings.
One of the shortcomings of the junction forgings proposed in those prior patents lies in the diameter of the forgings in the connection zone thereof is reduced so as to correspond substantially to the diameter of the inner pipe. This leads to a very large change in the second moment of area of the cross-section of the PiP between the main zone of said PiP and the connection zone between two unit lengths of the PiP, thereby creating a point of weakness at each of said connections by welding between two forgings, the zone of said welding then being particularly sensitive to fatigue phenomena, both during laying and throughout the lifetime of the pipe.
In order to avoid that zone of weakness and to conserve a second moment of area for the cross-section that is substantially constant, it is possible to increase the wall thickness of the forging throughout the zone situated between the solid portion of said forging and the chamfered zone where welding is performed. However that would generally require said thickness to be almost doubled. With pipes of large diameter or that are to be laid in great depths, welding then becomes problematic because of the very great thickness of steel, which thicknesses may reach 40 millimeters (mm) to 50 mm, thus requiring welding methods that are very difficult, and under certain circumstances that are practically impossible without defects, given the dynamic effects at sea on the mass of molten steel. Furthermore, since said welding is performed on board laying ships, where such ships present an extremely high hourly cost, the cost of the installation becomes prohibitive and the risks of failure are considerable because of the complexity of said welding operations performed on site.
It is then preferable to use the method described in patent FR-2 751 721 which consists, in a particular way of making the ends of a PiP, associated with a particular way of reinforcing the connection zone between two unit lengths of PiP by means of a sliding sleeve with little clearance relative to the outer pipe, said sliding sleeve being secured to said outer pipe by adhesive. That disposition makes it possible locally to increase the second moment of area of the cross-section so as to limit the stresses in the connection zone between two unit lengths of PiP, however it requires several mechanical parts to be fabricated that are complicated to assemble together and that require a relatively difficult connection operation to be performed. In addition, the proposed adhesive remains subject to creep and deteriorates over thermal cycling of the kind to which the pipes are subjected during their lifetime of 20 years to 30 years. Finally, that type of adhesive cannot be provided in reliable manner for bottom-to-surface connections, since the dynamic effects of the swell and of current on the pipe suspended between the sea bottom and the floating support quickly degrade the plane of adhesion, leading to rapid and excessive fatigue over the connection zone of the PiP.
Thus another problem of the present invention is to make a connection between lightweight insulated pipes of the PiP type in which the connection zones between two unit lengths are reinforced so that the stresses generated during laying are minimized.
More precisely, another problem posed is to make a connection between unit lengths of PiP type coaxial pipe assemblies that is improved so as to make it easier to implement the connection means and to perform connection operations, in particular by optimizing laying equipment, and in which the connection zones between two unit lengths are reinforced, so that the stresses generated during laying are minimized and so that the fatigue behavior in bottom-to-surface connections is radically improved.
It is known to make pipes completely out of composite material. However they are generally of high cost and present the major drawback of being buoyant under certain situations, which means that they need to be weighted down by adding external mass so that they stay in position after being laid, or even so as to make laying possible.
In addition, and above all, pipes made of composite material are:                either made as continuous lengths that can therefore not be made by connecting together unit assemblies of coaxial pipe on board a laying vessel at sea;        or else made by connecting together unit elements on board a laying ship at sea; but under such circumstances, connecting together said unit elements of composite material presents major complications due to the stresses in the connection zone between two successive lengths. Junction parts made out of composite material are not strong enough, given the stresses in the vicinity of the connection. Furthermore, junction forgings made of steel are not easily fitted to coaxial pipe elements made of composite material.        
Finally, when it is desired to convey a corrosive fluid in the inner pipe, e.g. such as H2S, it is necessary to provide an extra thickness of stainless steel that is suitable for withstanding such corrosion, thereby firstly further increasing the weight of the pipe, and secondly being complex and expensive to make.
A final problem on which the present invention is based is thus that of providing a PiP pipe assembly comprising an inner pipe whose inside wall is made of special anti-corrosion steel without making the pipe heavier and in a method of fabrication that is simplified.
EP 0 635 667 discloses simple undersea pipes made of steel that can be said to be “bound” because they are reinforced by composite material deposited by winding glass or carbon fibers over the length of each of the elements apart from their end portions, said end portions being reinforced after said elements have been interconnected on board the laying ship, within the J-lay tower, so as to present substantially uniform strength all along said pipe. With that type of composite pipe, the fibers are wound at an angle lying in the range 65° to 87°. The steel pipe must be capable of fully withstanding the bottom effect, while the radial bursting effect is taken up essentially by the composite reinforcement. During laying, all of the laying tension is taken up by the steel pipe, the composite material reinforcement not contributing significantly to the traction strength of the steel and composite pipe. In that type of single-walled steel and composite pipe, the composite material performs a binding role only, and the steel pipe must present sufficient strength of withstand traction forces during laying and also the bottom effect when the pipe is under pressure, thus requiring thickness that is indeed likely reduced but still relatively large for the steel wall of said pipe, and in particular greater than more than 50% of the thickness of the non-reinforced ends of the pipe or of a steel pipe of the same diameter for taking up the same bottom effect. The reduction in weight that is obtained by such a disposition is therefore fairly limited. In addition, that type of pipe does not present sufficient thermal insulation properties.
The various problems on which the present invention is based are solved by providing a coaxial pipe element in which the inner pipe is made of a composite material comprising a very thin inner tube of metal that is covered by inorganic fibers wound around said inner tube, i.e. a tube having thickness that is less than 50% that of a steel pipe of the same diameter needing to withstand the same bottom effect in full.